For many applications in optical ranging and optical imaging using interferometric based techniques, it is necessary to use a scanning optical delay line as a component of the measurement apparatus. A conventional scanning optical delay line produces a delay by propagating the optical beam through a variable path length. Such a conventional delay line produces a change in phase delay and group delay which is determined by the geometric path length divided, respectively, by the phase velocity and group velocity of light in the medium of propagation.
Previous optical delay scanning devices have largely relied on scanning of the optical path length in order to achieve delay scanning. Devices using linear actuators, spinning mirrors or cam-driven linear slides have been demonstrated. Most current mechanical scanning optical delay lines are not rapid enough to allow in vivo imaging owing to the presence of motion artifacts. Piezoelectric optical fiber stretchers that allow rapid scanning have been demonstrated but they suffer from high power requirements, nonlinear fringe modulation due to hysteresis and drift, uncompensated dispersion mismatches, and poor mechanical and temperature stability. In addition the concept of using a system of diffraction gratings and lenses has been demonstrated for stretching and compressing short optical pulses, pulse shaping and phase control. A combination grating and lens device has been demonstrated for scanning delay in a short pulse autocorrelator. The device produces a change in group delay by angular adjustment of a mirror, however, it does not permit the phase delay to be adjusted independently of the group delay.
Such delay lines are useful in performing Optical Coherence Tomography (OCT). OCT is a relatively new optical imaging technique that uses low coherence interferometry to perform high resolution ranging and cross sectional imaging by illuminating the object to be imaged with low coherence light and measuring the back reflected or back scattered light as a function of time delay or range. Optical ranging and imaging in tissue is frequently performed using a modified Michelson or other type interferometer. Precision measurement of optical range is possible since interference is only observed when the optical path length to the scattering features within the specimen and the reference path optical path length match to within the coherence length of the light.
The axial reflectance of structures within the specimen is typically obtained by varying the reference arm length using a mechanical scanning linear galvanometer translator and digitizing the magnitude of the demodulated interference envelope or direct digitization of the fringes. A cross-sectional image is produced by recording axial reflectance profiles while the position of the optical beam on the sample to be imaged is scanned. Such imaging can be performed through various optical delivery systems such as a microscope, hand-held probe, catheter, endoscope, or laparoscope.